An inductive proximity switch of that type is described in European patent applications no. EP 0 936 739 A1 and EP 0 936 741 A1. In that proximity switch, a coil is supplied with periodical transmitting current pulses with a period exceeding the pulse length. By means of these transmitting current pulses, a voltage is periodically induced in an object to be detected. In turn, a voltage is induced in the coil subsequently after the end of a transmitting current pulse by means of the decaying current that flows in the object due to the voltage previously induced therein. The useful signal is then obtained by a suitable electronic circuit which responds to the voltage that is received by the coil in such a manner.
A disadvantage of the proximity switch is that its magnetic circuit is prone to couple with perturbing fields from the environment. Depending on the frequency band, the interaction of the proximity switch with external fields can downgrade or destruct its measuring performance. For instance, external fields in the lower frequency range up to 500 Hz and in the medium frequency range up to 10 kHz are often present in favored application areas of such a proximity switch. Most critical are external fields within the frequency band of the signal to be detected and amplified by the electronic circuit of the proximity switch—typically in the lower and/or medium frequency range—and an elimination of these perturbations is essential for the basic functionality of the proximity switch. More generally, an elimination of perturbations over the whole frequency spectrum is highly desirable for yielding an output signal of good quality.
The impact of external fields may be partially or fully suppressed—primarily at the lower frequency range—by providing the transmitting current pulses with a periodically reversed polarity, as described in European patent application no. EP 0 936 739 A1. In this approach, however, perturbing fields with larger amplitudes or frequencies in the medium or higher frequency range may not be fully eliminated by the polarity reversal.